How do I create a range from a begin and end iterator?

Question

I have an object with functions for getting the begin and end iterators:

const_iterator err_begin() const
const_iterator err_end() const

Because they are not named begin and end, I cannot pass my object directly to functions in range-v3.

Is there a simple wrapper I can use to make this object work with the range-v3 library?

For example:

auto hasErrors = !empty(something(x.err_begin(), x.err_end()));

Add `begin()` and `end()` that call these two functions, respectively? — Sam Varshavchik, Dec 21 '16 at 17:06
The object is part of a library. I cannot change the method names or add new methods. — sdgfsdh, Dec 21 '16 at 17:07
Create the simple wrapper you are talking about that maps the begin and end calls to the ones you want. — mascoj, Dec 21 '16 at 17:10
Add overloads to `begin` and `end` functions for your object. They don't need to be methods — rocambille, Dec 21 '16 at 17:10
`class_name::const_iterator begin(class_name& obj) { return obj.err_begin(); }` — Pete Becker, Dec 21 '16 at 17:10

score 17 · Accepted Answer · edited May 25 '23 at 15:18

17

In C++20 onwards, you're looking for subrange:

auto hasErrors = !empty(std::ranges::subrange{x.err_begin(), x.err_end()});

If you're still using Ranges-v3, you'll want iterator_range:

auto hasErrors = !empty(ranges::make_iterator_range(x.err_begin(), x.err_end()));

edited May 25 '23 at 15:18

Toby Speight

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answered Dec 21 '16 at 17:12

Brian Rodriguez

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The documentation for `iterator_range` lives [here](https://ericniebler.github.io/range-v3/structranges_1_1v3_1_1iterator__range.html). Just call `ranges::make_iterator_range(x.err_begin(), x.err_end())`. – Eric Niebler Dec 21 '16 at 17:29
I cannot find `iterator_range` and `make_iterator_range` from the code anymore. Have they been renamed? – kaba Mar 13 '17 at 23:00
@EricNiebler, why do we need `iterator_range`, while we can simply create a `range` of iterators?.. – Mikhail Mar 18 '18 at 16:27
1

Ah, are you using the Microsoft fork of range-v3 for MSVC? In that (very out-of-date) fork, `iterator_range` was called `range`. It isn't any longer. – Eric Niebler Mar 19 '18 at 20:25
4

For future readers: in C++20 `std::ranges::subrange` seems to have replace `ranges:make_iterator_range`. – Werner Henze Jul 26 '21 at 21:16

score 4 · Answer 2 · answered Dec 21 '16 at 17:10

You clarified that the class in question is part of a library that you cannot change. Fine. Create a facade class:

class FacadeClass {

      const RealClassWithErrBeginEnd &r;

public:

      FacadeClass(const RealClassWithErrBeginEnd &r) : r(r) {}

      auto begin() const { return r.err_begin(); }
      auto end() const { return r.err_end(); }
};

This should be good enough to fool most code that expects a container. In the worst case, you might need to provide additional typedefs in the facade, i.e. value_type, etc...

This type is not [Regular](http://stackoverflow.com/a/14000046/195873). It will not play nicely with the rest of the range-v3 library. Brian's answer below is the correct answer. (I'm the author of range-v3, FWIW.) — Eric Niebler, Dec 21 '16 at 22:45

score 0 · Answer 3 · answered Dec 21 '16 at 17:18

boost::make_iterator_range will do the right thing. Now add a little ADL and we find that one free function solves all our problems:

#include <vector>
#include <iostream>
#include <string>
#include <boost/range.hpp>


// simulate the library class
struct X
{
    auto err_begin() const { return errors.begin(); }
    auto err_end() const { return errors.end(); }

    std::vector<std::string> errors;

};

// provide a generator to build an iterator range
auto errors(const X& x)
{
    return boost::make_iterator_range(x.err_begin(), x.err_end());
}

// do things with the iterator_range
int main()
{
    X x;
    for (const auto& err : errors(x))
    {
        std::cout << err << std::endl;
    }

    std::cout << empty(errors(x)) << std::endl;
}

How do I create a range from a begin and end iterator?

3 Answers3